During drilling of a well, completion thereof, and subsequent remedial services conducted after the well has been placed in service, all procedures may require use of tools lowered on wirelines. A wireline is defined for purposes of this disclosure as a woven or wrapped, multistrand wire wherein the strands have a lay or lay distance to thereby define externally observable helixes between adjacent strands. In well logging, the wireline comprises externally armored electrical cable having a plurality of insulated inner conductors. The well logging tools which are typically lowered on a wireline are ordinarily lowered to specified depths. Different tools may be required to be lowered in multiple operations to the same or different depths wherein the depth measurements must be accurate and repeatable for multiple operations to be conducted. For instance, assume as an example that a hydrocarbon producing sand has been located that is 20 ft. thick and is located at a depth of 20,000 ft. It will be understood that a one percent (1.0%) error in the measurement of the position of a wireline supported tool represents an error of 200 ft. at that depth, an error which is ten times greater than the thickness of the producing formation of interest. Clearly, it is necessary that accurate repeatable wireline depth measurements be made so that the multiple steps in servicing the well can be carried out accurately.
A similar requirement may exist in conducting different operations by a single tool at multiple depths. For instance, a logging tool may be used to record multiple formations as the tool is retrieved from a depth of 20,000 ft. Some of the formations may be only a few feet in thickness and hence, it is important that the depth measurements of the tool as it traverses the well borehole by accurate so that all the measurements and hence all the observed formations can be accurately located relative to well depth.
Examples of depth measuring equipment used in the past and representative of the prior art, include the placement of magnetic marks on the wireline at specified intervals along the length. It is necessary to accurately control tension of the wireline while marking the line since to insure that the magnetic marks are properly spaced. Errors in measurement may arise from variations in tension while marking the wireline. In making measurements with a wireline having marks at 100 foot intervals, some additional type of measuring device is also required to cover the possibility that measurement to an intermediate point (other than multiples of 100 ft.) might be accomplished. This additional measuring device is susceptible to erronous measurement. Moreover, magnetic marking of a wireline requires a calibrated measurement instrument to provide accurate measurement. In making depth measurements, tension variations require correction in relation to the measured tension versus calibration tension. Another type of arrangement of measuring device is an incremental encoder driven by a sheave which provides pulses as the sheave is rotated when the wireline passes over the sheave. Thus, sheave rotation is assumed to be proportional to wireline length. Errors in measurement can arise from wireline-to-sheave slippage of the marked 100 foot increments which could produce further error. In this type of system, one presumes that the sheave does not slip and maintains an accurate and controllable circumference. But, slippage is possible. Also, mud or ice (in cold environment) can accumulate and build up on the wireline and hence change the diameter of the wireline or the sheave or both. Errors arising from tension measurement translate into depth measurement errors. The present apparatus provides a positive wireline engaging mechanism which thereby yields improved depth measurements. The wireline is typically constructed with a central core which may include insulated conductor wires. It is typically formed with multiple armor strands wrapped around the exterior to form a surrounding sheath defined by strands laid against one another in helix form. The pitch of the wireline is determined in part by the diameter, number of strands, and the tightness of the twist. Wirelines are typically described in terms of diameter, pitch and number of strands. Such a wireline is measured by the method and apparatus of the present invention. This apparatus and method have advantages over the prior art. Included among examples of the prior art are U.S. Pat. Nos. 3,520,062 and also, 4,179,817. The present apparatus positively follows the wireline as it passes into the borehole, tracking the wireline by following the helix on the exterior. This imparts rotation to a helix follower having the form of a wheel with a relatively sharp edge inserted into the helix for following the helix. As the wireline passes through the equipment, and as the revolution of the helix pass the equipment, the revolutions are noted by forcing the helix follower to rotate about the wireline. The follower is supported on a transverse bearing system. As it rotates, it drives a chain or toothed belt engaging a sprocket with an electrical encoder forming an output signal proportional to revolutions of the helix follower. Suitable guide rollers supported by a mounting means complete the system so that it can be positioned at the wellhead or on a logging truck in near proximity to the wireline which is lowered into the well to support a tool.